The advantages of the tiltrotor concept are many. The ease with which the aircraft can be converted from one flight mode to another enhances its maneuverability and permits the aircraft to be configured to meet mission requirements. Also, the unique tiltrotor flight mode provides a flight envelope that overlaps the helicopter and airplane flight envelopes, increasing mission flexibility. Operating as a VTOL aircraft, it can take off like a helicopter and deliver payloads on less fuel than consumed by a helicopter. Takeoff and landing terminals can be small, making tiltrotor aircraft ideal for intercity commuter travel. In the STOL mode, tiltrotor aircraft are ideal for long distance transport of heavy cargos into remote areas. Only short runways are necessary.
Tiltrotor aircraft combined features of helicopters and fixed-wing aircraft. They have the vertical takeoff and landing ability of the helicopter and the cruise speed, range, and fuel economy of fixed-wing aircraft. Tiltrotors achieve this by the use of rotors that operate like helicopter rotors during takeoff and landing, then tilt to horizontal thrust to act like turboprop propellers during cruise.
The unique feature about these aircraft is the two large multi-bladed proprotors mounted at the tips of the wings. For takeoff, the proprotors and their engines are rotated to the straight-up position where the lift developed is entirely propulsive. Once off the ground, they have the ability to fly in one of three different modes. It can fly as a helicopter, in the partially converted airplane mode. Also, they can convert rapidly from the helicopter mode to the airplane mode. This is accomplished by continuous rotation of the proprotors from the helicopter rotor position to the conventional airplane propeller position.
During the 1920s and 1930s, the numerous innovative flying machines devised included several concepts that were expected to provide vertical takeoff capabilities. One of these was developed in the U.S. by Henry Berliner in the early 1920s. This design resembled a fixed-wing biplane of the period, except that it had a large diameter fixed-pitch propeller mounted on a vertical shaft near the tip of each wing. For forward flight, the shafts would be tilted forward. Reports indicate that the Berliner helicopter achieved forward speeds of about 40 mph. While the propellers were not designed to convert fully to the conventional airplane mode, the Berliner side-by-side helicopter was an early example of the rotor arrangement used on current tilt rotor aircraft.
Another design conceived to provide vertical lift and forward flight is the "Flying Machine" for which George Lehberger was issued a patent in September 1930. His approach contained the basic concept of the tilt rotor aircraft, that is, the use of a relatively low disc loading thruster (propeller) that can tilt its axis from the vertical (for vertical lift) to the horizontal (for propulsive thrust). While there was no attempt by inventor George Lehberger to develop this vehicle, it would be expected to encounter performance, loads, structural dynamics, and control deficiencies if built as indicated in the patent illustration. The vectored thrust low disc loading VTOL aircraft required many technology advancements before it would be a practical aircraft type.
In the late 1930s, a British patent was issued for the Baynes Heliplane which resembled the configuration of the current tilt rotor aircraft. Inadequate financial backing prevented development work, leaving the exploration of tilt rotor technology to other engineers in the four decades that followed.
In Germany, the Focke-Achgelis FA-269 trail-rotor convertiplane project was initiated in 1942. This aircraft followed the moderately successful lateral twin-rotor helicopter, the Focke-Wulf Fw-61 flown in 1937. The FA-269 used pusher propellers that tilted below the wing for takeoff. This project was discontinued after a full-scale mockup was destroyed during a bombing in WWII. Years later, variants of the trail-rotor tilt rotor configuration would surface again in design studies at Bell and McDonnell Douglas.
The accomplishments of the German Focke-Wulf activities did not go unnoticed by the Americans. Two enterprising engineers, Dr. Wynn Laurence LePage and Haviland Hull Platt of the Platt-LePage Aircraft Company of Eddystone, Pennsylvania, became intrigued with the success of the German helicopter and decided to pursue the development of a viable helicopter in the U.S. with the same general arrangement of the Fw-61. The product of this work was the 1941 Platt-LePage XR-1A lateral twin-rotor helicopter. This aircraft inspired the design of a large (53,000-lb.) tilt rotor aircraft, which resembled the XR-1A configuration, but incorporated mechanisms that permitted the rotors to be tilted for forward flight. While Platt-LePage never developed their tilt rotor design, Haviland Platt applied for a patent for the concept on July 7, 1950, and was granted U. S. patent 2,702,168 on February 15, 1955.
The next significant appearance of the tilt rotor occurred in early 1947 when the Transcendental Aircraft Corporation of New Castle, Delaware, initiated work that led to the development of the Model 1-G tilt rotor aircraft. The founding principals of Transcendental were Mario A. Guerrieri and Robert (Bob) L. Lichten, who had been coworkers at the Kellett Aircraft Company. Bob Lichten had earlier worked for pioneer helicopter designers LePage and Platt and had become intrigued with the tilt rotor concept. His experience at Platt-LePage provided him a mission that he pursued for the rest of his life.
While at Kellett, Guerrieri and Lichten investigated the performance of a helicopter rotor acting as a propeller and, encouraged by the results, decided to demonstrate tilt rotor technology by independently building and flying a small, single-place experimental aircraft, the Transcendental Model 1-G. Lichten left Transcendental in 1948, and, in September 1952, Guerrieri sold his interests in the company to William E. Cobey, a Kellett Aircraft Corporation vibrations expert who continued the development of the Model 1-G. With some funding provided by a 1952 Army/Air Force contract for flight data reports and analyses. Hover testing of the 1750 lb. Model 1-G, the very first experimental tiltrotor plane, began on June 15, 1954. The plane barely flew. It was very fragile, and vibration was a very big problem. The Transcendental Model 1-G met an unfortunate end. After successfully completing more than 100 flights in a period of just over one year, including partial conversions to within 10 degrees of the airplane mode, an inadvertent reduction of the rotor collective pitch while flying with the rotors tilted forward led to a crash into the Chesapeake Bay on July 20, 1955. Although the aircraft was destroyed, the crash occurred near land in shallow water, which allowed the pilot, who sustained minor injuries, to wade ashore [other accounts state that the accident was fatal to the pilot].
A second, improved, Transcendental tilt rotor aircraft, the 2,249 lb., two-place Model 2, was subsequently developed by Cobey but funding limitations resulting from the withdrawal of Air Force support prevented the expansion of the flight envelope, and the program was terminated in 1957.
The XV-3, built in 1953, was an experimental aircraft that flew until 1966. It proved the fundamental soundness of the tiltrotor concept and gathered data about technical improvements needed for future designs. Although still very fragile, the XV-3 was the first plane to successfully convert from vertical flight to horizontal flight. That conversion, though, was the XV-3's only feat. It didn't carry passengers or freight.
In the early 1970s a review was conducted of the status of technology related to the tilt-rotor concept. The problems associated with the XV-3 aircraft, notably the aeroelastic (whirl flutter) instability of the rotor at high forward speeds, had been thoroughly researched during the intervening years in the wind tunnel, and a new rotor had been designed and tested at large scale. The results of those tests showed that it should be possible to design a tilt-rotor aircraft with maximum cruise speeds in excess of 350 knots. In 1972 the technical program at Ames was focused on the development of a research aircraft; the size of the aircraft was chosen so-as to permit testing in the 40- by 80-ft wind tunnel before the aircraft was taken into forward flight, and sufficiently large to permit extrapolation of results to an aircraft of useful operational size. The resulting XV-15 Tilt-Rotor Research Aircraft was developed between 1972 and 1977. With funding from NASA and the US Army, Bell Helicopter Textron started development of the XV-15 in 1972. Two aircraft were built to prove the tiltrotor design and explore the operational flight envelope for military and civil applications. The Army's motivation for going into this project was need for a medivac airplane capable of going straight from the battlefield to the hospital. NASA and the army funded further development of the tiltrotor for 10 years at a total cost of about $100 million. The XV-15s demonstrated excellent handling, low pilot workload, and good ride qualities; they continue to be used as experimental testbeds.
The V-22, using experience gained from the XV-3 and XV-15, Bell Helicopter Textron and Boeing Helicopters began developing the V-22 "Osprey" in 1981. This twin-turboshaft military tiltrotor aircraft. Six flying full-scale development aircraft were to be built; four had flown at year-end 1990.
The Civil Tiltrotor Development Advisory Committee Act of 1992 directed the Secretary of Transportation to establish a Civil Tiltrotor Development Advisory Committee in the Department of Transportation to evaluate the technical feasibility and economic viability of developing civil tiltrotor aircraft and a national system of infrastructure to support the incorporation of tiltrotor aircraft technology into the national transportation system.
NASA's Civil Tiltrotor research is developing the most critical vehicle technologies for a civil tiltrotor. Because a tiltrotor airliner can take off and land vertically, there are two major benefits of a CTR. A tiltrotor aircraft can add additional capacity to an airport and reduce delays. Significant reduction of door-to-door trip times for passengers, by circumventing ground and air congestion. Expansion of the capacity and reduction of the runway congestion at the busiest airports by permitting some short-haul traffic (trips of less than 500 miles) to shift to tiltrotors, freeing runway space for larger aircraft.
There were two problems that had to be overcome before this vehicle could be considered for commercial service. Noise was a key issue, especially in the approach to the airport terminal areas. Another was continued safe operation of the aircraft if one engine became inoperative, a condition required for certification by the FAA. A certified civil tiltrotor transport would also be required to fly in adverse weather while maintaining airline levels of safety during approach and landing.
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